The present invention relates to a pneumatic brake power booster for automotive vehicles including a booster housing with an interior which is subdivided by a movable wall into a first chamber (vacuum chamber) and a second chamber (working chamber), and a control housing accommodating a control valve which controls a pneumatic pressure difference acting upon the movable wall, the control valve including at least two sealing seats that cooperate with an elastic valve member, and being operable by an actuating rod, on the one hand, and irrespective of the actuating rod by an electromagnet, on the other hand, the armature of the electromagnet cooperating in a force-transmitting way with one of the sealing seats, and the valve member confines a pneumatic chamber in the control housing which can be acted upon by the pneumatic pressure prevailing in the second chamber through passages provided in the sealing surface of the valve member.
International patent application No. WO 94/11226 discloses a brake power booster of this type. A disadvantage of the prior art brake power booster is the effect of an annular surface on the valve member of the control valve, more particularly, in the release action with the electromagnet activated. The annular surface is limited, on the one hand, by the radially outward sealing lip of the valve member and, on the other hand, by the third sealing seat which bears against the sealing surface of the valve member. The annular surface which is acted upon by the pneumatic differential pressure prevailing in the housing of the brake power booster during its operation generates a force component. The force component counteracts the independent actuating force generated by the electromagnet and must be compensated by a corresponding rating of a piston rod return spring which retracts the valve piston of the control valve.
Therefore, an object of the present invention is to disclose measures which permit eliminating the disturbing effect of the above-mentioned force components.